Current generator



March 9, 1965 M. euro-r CURRENT GENERATOR 3 Sheets-Sheet 1 Filed April 18, 1960 pvvsyron MAURICE GUIOT' March 9, 1965 M. GUIOT CURRENT GENERATOR 3 Sheets-Sheet 2 Filed April 18, 1960 INVENTOR Mam/c5 GU10? Wm {M 1/ Fig.4.

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M. GUIOT CURRENT GENERATOR March 9, 1965 3 Sheets-Sheet 5 Filed April 18, 1960 INVE/WOR MAUR 1 CE Gum r v United States Patent P 3,173,075 CURRENT GENERATOR Maurice Guiot, Paris, France, assignor to Berex Establishment, Vaduz, Liechtenstein Filed Apr. 13, 1960, Ser. No. 22,786 Claims priority, application France, Apr. 24, 1959, 793,046; July 10, 1959, 799,864- 3 Claims. (Cl. 322-40) The present invention relates to improvements in generator apparatus and notably in those incorporated in the electrical equipment of automotive vehicles.

Electrical equipments of automotive vehicles are becoming more and more important and one feature characterizing this development is the recent trend towards a general use of 12-volt equipments instead of the hitherto conventional 6-volt equipments, but it is evident that new solutions must be brought forthwith to the problem of the power output, increment contemplated for future equipments.

The conditions of operation of automotive vehicles have developed considerably in. the last few years, on the one hand on account of the heavier traihc and on the other hand by reason of the greater flexibility of internal combustion engines which affords a wider range of engine speeds.

Whereas a few years ago the average speed of a vehicle in town streets was of the order of half the aver-age road speed, the ratio of town speed to road speed is now about 1:4, the town speed being reduced by traffic conditions and/or town regulations to about 20 mph, whilst road speeds are frequently of the order of 75 mph. with most modern cars.

This speed discrepancy will certainly increase due to the substantial improvements in the design of internal combustion engines and to the traffic becoming heavier everyday.

Under these conditions, manufacturers of current-gencrating equipments for vehicles are confronted with the necessity of designing and developing the generator and regulator assembly in such a way that it will provide an adequate supply of current to the vehicle in both extreme cases; in other words, this assembly must be capable of delivering its rated output from 20 mph up in top gear or direct drive (thus determining hhe size of the genera-tor capable of supplying an adequate current at this speed), and also to withstand without any deleterious consequence a velocity of rotation four time higher; anybody conversant with the art will readily understand that tins dual and technically contradictory requirement sets very difiicult problems in connection both with the generator itself and with the regulator controlling same.

it appears that present equipments have attained the utmost limits or" technical possibilities according to con ventional practice; thus, they are characterized by the use of a generator or dynamo over-dimensioned for all conditions of operation other than those encountered in large towns where the average speed is relatively low, and operating under bad conditions in the open country at high speed.

As an example, the case of an equipment mounted on a mass-produced light car exported throughout the world may be cited. This car has a speed of 14.3 mph. per 16-00 rpm. of the engine in top gear rep-resenting 1,300 engine rpm. at 18.65 mph. and 5,000 rpm. at top 3,173,675 Fatented Mar. 9, 1965 speed, is, 71.5 mph. The lid-ampere equipment has a l2-volt rating and comprises a generator or dynamo having a contact speed of 1,500 r.p.m., the rated fulloutpu-t speed being 2,500 rpm. and the maximum speed of operation 10,000 rpm; (corresponding to the conditions set forth hereinabove) with an engine to dynamo transmission ratio of 1:2.

This type of equipment requires very careful design and construction; it is therefore costly while being attended by risks of accidents in operation owing to the very high rotational speed and also to the very high ratio of the regulation speeds.

Various solutions have been proposed with a view to solve the problem set forth herein; thus, two practical solutions based on different principles may be reminded hereinafter:

The first solution consists in utilizing a generator designed for operation at road speeds but driven from a pulley in which a reducing gear responsive to the centrifugal force is incorporated between the pulley groove and the armature shaft, this gear providing two reducing ratios, one for town driving and the other for road driving. This system, widely used in the United States on special vehicles such as tam's, ambulances, police cars and the like, is extremely costly and delicate and therefore has not been mounted on mass-produced vehicles.

The other solution consists in substituting an alternator for the DC. generator, as the former afifords a greater range of speeds than dynamos, due to the suppression of commutation problems. Although this solution actually solves one problem, it leaves unchanged the size problem (due to the minimum speed, as in the case of the dynamo) as well as the regulation problems.

it is the essential object of this invention to provide a novel and logical solution to the above-mentioned problems of current enerating equipments for automotive and other vehicles, whether the generator is a dynamo or an alternator.

According to this invention the current-generating equipment, notably for automotive vehicles, comprises a driving shaft, a current generator comprising an armature and a stat r, the armature being rotatably driven from said shaft, an electromagnetic coupling of the variableslip type comprising a field winding, a driving armature and a driven armature, said driving armature being rotatahly rigid with the driving shaft, said driven armature being rotatably rigid with the ge orator armature, the field windings of said electromagnetic coupling comprising a permanent magnet and an electromagnet, the magnetic flux of the permanent magnet being suthcient for rot-stably driving the driving armature of the coupling and therefore the armature of the generator when the electromagnet is not energized, and means for varying the energizing curre t fed to the elcctromagnet as a iunction at the speed of the generator armature so that the amount of slipping in the coupling will vary as a function of the driving shaft speed, and that the speed of the generator armature will either lie between two predetermined limits or remain constant.

Due to the provision of the coupling it is possible to select with an exceptionally high efficiency a gear ratio such that on the one hand the generator will remain as economical as possible for town use and that, on the other hand, its design and type of regulation will be ex tremely simplified by virtue of the constant or moderately variable speed at which it is operated.

According to a specific embodiment the coupling ping may be controlled with a view to maintain the generator armature at a constant speed so that, in the case of an alternator, it is possible to directly use alternating current at a constant frequency for energizing the circuits necessitating this type of current.

The slipping control may also be combined with the control of the generator excitation with a view to adapt the slipping under the best possible conditions to the extremely variable power output to be delivered by the generator under the different conditions of operation and according to the state of charge of the storage battery.

With the foregoing and other objects in view, the invention resides in the novel arrangement and combination of parts and in the details of construction hereinafter described and claimed, it being understood that change in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.

Other objects and advantages Will become apparent from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a diagram illustrating the principle of the current generator equipment according to this invention.

FIGURE 2 is a longitudinal section taken along the axis of a coupling and generator assembly according to this invention.

FIGURE 3 is a cross-section taken upon the line IIIIII of FIG. 2.

FIGURES 4 to 7 are wiring diagrams illustrating different forms of embodiment of regulators constructed according to the teachings of this invention; and

FIGURE 8 is a longitudinal section illustrating a modified embodiment of the current-generating equipment of this invention.

In FIG. 1, the generator 1 is rotatably driven from a driving shaft 2 through the medium of an electromagnetic coupling 3. The driving shaft 2 drives a bell-shaped armature and the driven armature rigid with the generator shaft 4 consists of a permanent magnet 21 and an electromagnet 22.

A regulation device may be mounted on the shaft 4 of generator 1 which reacts as a function of the generator speed upon an element s having variable electrical characteristics which controls the energizing current flowing through the electromagnet winding 22.

In FIG. 2, the electromagnetic coupling 3 comprises essentially an armature ill rotatably driven at a relatively high speed and without slipping from the engine of the vehicle by means of a grooved pulley 12, this armature constituting the driving element of the electromagnetic coupling. This bell-shaped member in which the eddycurrents are concentrated is made from adequate magnetic steel and may be easily pressed and accurately machined with a view to rotate same at a speed of the order of 10,000 rpm. The driving V-grooved pulley 12 may be mounted on a threaded shaft portion 13 permitting, in addition, the adjustment of the ball-bearings 15. A spider-shaped pressed member 16 of blued steel acts as a play take-up element and minimizes possible gyroscopic effects.

A gap surface 17 is provided which in this example is of a cylindrical configuration, but if desired a tapered gap surface may be provided, for example by turning, in order to reinforce the torque thus transmitted, due to the reduction in the gap 17. In this case the driving armature ill urged ,by the spring 16 is simply allowed to recede, by unscrewing the pulley l2 and finally locking same by means of the nut 18.

The driven portion of the electromagnetic coupling comprises two cast-iron or steel flanges or plates 19, 20 formed with integral pole pieces or fingers 19a, 20a exsliptending alternately in the gap surface as in known couplings.

According to this invention, two complementary excitation systems are provided. The first system consists of a permanent magnet 21 made generally from a so-called ferroxide material or equivalent magnetic material creating a flux sufficient to provide a driving torque having just the value required for exciting the generator if this type of excitation is contemplated, or providing the no-load losses or the minimum output in the case of a machine wherein the excitation is effected through permanent magnets.

The second excitation system comprises an electromagnet the field-winding 22 of which is energized from a DC. source. This electromagnet is adapted to deliver a sufficient flux adding itself to the flux so that the torque transmitted will be capable of driving the generator under full-load conditions at a speed of 1,000 to 2,000 r.p.m.

The coupling comprises furthermore a ventilating turbine 24 of the axial-flow type which has the dual function of cooling the generator and coupling assembly, wide ventilation apertures 25 being provided to this end in the lower portion of the frame or case 9.

For operating the generator at a constant speed the energizing current of winding 22 may be controlled by means of a centrifugal regulator of known type of which the movable member acts upon the arm of a rheostat connected between the source of energizing current and the Winding, or by means of any other electromechanical or electronic device the control magnitude of which is the frequency given either by the generator, in the case of an alternator, or by a pulse generator associated with the armature.

FIGURE 4 illustrates a typical example of an electronic regulator. In this example the current flowing through the electromagnet winding 22 of the electromagnetic coupling 3 is controlled by a pilot transistor 40. The brush 26 in sliding contact with the commutator 27 is connected to the collector electrode of transistor 4% the emitter electrode of which is connected in turn to a source of positive DC. The transistor base is grounded through the circuit comprising the resistor 41 and capacitor 42 in parallel. The base of the transistor has also connected thereto through a diode 4d a Winding 43 carried by a magnetic circuit i t having pole pieces, the poles of a magnetized core 45 moving past these pole pieces, as shown.

During the operation of the machine, if the speed of generator 1 tends to increase, the voltage of the oscillations delivered through the winding 43 increases and causes the base voltage of transistor to increase accordingly. Thus, the emitter to collector current drops and therefore the excitation of the electromagnet winding 22 decreases as well. Then the slipping increases and the generator speed becomes stabilized.

FIG. 5 illustrates another typical form of embodiment of the regulation system, wherein the generator 1 consists of an alternator. The oscillations generated by the alternator I are fed to a potentiometer 47 having its sliding contact connected to the base of a transistor 48 acting as a rectifier. The rectified oscillations are integrated by a capacitor 49 and the resultant voltage controls another transistor 5%? controlling the current in the electromagnet winding 22.

According to a specific form of embodiment of a regulation system a centrifugal regulator 23 mounted in a cylindrical extension 8 of the generator frame may be used. The device for delivering current to the coupling eleotromagnet comprises a terminal 10 associated with the brush holder 26 and connected to the positive terminal of a DC, source, for example a storage battery, through the medium of the key-operated ignition switch of the vehicle. A brass or bronze commutator ring 27 is secured on an insulating member 25 rigid with the shaft 4 so as to be connected to the positive terminal of the installation. The Winding 22 of the electromagnet is connected through a conductor 29a to the commutator 27 and through another conductor 29]) to the contact-breaker 30. To this end, the shaft 4 is formed with a central passage l4 extending throughout its length.

The first conductor 29b constituting the ground return wire or outlet wire of the electromagnet winding is part of a circuit periodically opened at each revolution by the contact-breaker 30. Contrary to current practice of ignition breaker construction, in this case it is the contact-breaker that is driven by the rotary insulating case 31, the circuit-opening cam 32 being fixed and secured on the end cover 33 (see FIGS. 2 and 3).

From the foregoing it is clear that the time 2 during which the electric contact is established between the contact-breaker 30 and the ground, that is, between the cam follower 35 and the lobe of cam 32, varies as a function of the velocity of rotation of the generator shaft. This time t; is maximum at low velocities of rotation, and will be strictly consistent with the cam-controlled up and down movements if an adjustable insulated stop 34 (see FIG. 3) is provided to limit the movement of the contact-breaker 3t and prevent the follower 35 from contacting the lower portion of the fixed cam 32 during the time t contemplated for the open-circuit period.

Each revolution of the generator shaft will thus close during the time 1 the energizing circuit of the coupling electromagnet and open this same circuit during a time period t such that t +t =Z, that is, the time necessary for the generator to perform a complete revolution.

According to the torque required for driving the generator and to the different parameters intervening in the coupling, a cam giving pulses t sufiicient for driving the generator under full-load conditions at a speed of, say, 2,000 r.p.m., may be designed. If desired, a disc may be substituted for the cam, the circuit being opened not at each revolution but under the influence of the centrifugal force.

Tests are necessary for determining the value of the sparloarresting resistor 36 mounted in the insulating case 31 and connected across the contact-breaker 30 and the ground in order to absorb the break current caused by the electromagnet reaotance.

This resistor 36 wound on a mica support and properly cooled by the rotation of the insulating case 31 carrying the contact-breaker plays on the other hand a prominent part in the regulation. As the pulses of current i during a time t are followed by pulses of reduced current value i it will be readily understood that the average excitation current in the coupling and, therefore, the torque transmitted to the generator, may be varied at will by modifying the value of this resistor.

The combination of the following three elements, that is, the cam contour 32, the adjustment of stop 34 and the value of the interchangeable resistor 36 makes it possible to provide any desired conditions of regulation stability throughout the range of speeds at which the machine is operated, inasmuch as the permanent magnet 21 will reduce approximately 50% of all the instability factors resulting from the electromagnet energization.

In case of racing, the breaker arm 30 will be separated completely from the cam 32 so as to positively stop the energization of the electromagnetic coupling; under these conditions, the speed of the generator retarded by the power output delivered to the charging circuit will be reduced instantaneously.

Instead of causing the generator to rotate at a constant speed as before, it may be advantageous to admit of a certain speed variation as a function of load and of the voltage requirement of the generator, and also with a view to compensate the speed variation resulting from the temperature of the generator windings.

This result may be obtained by substituting for the centrifugal regulator controlling the current in the coupling winding 22 a regulator of which the adjustment '6 parameters are the voltage, the current or any other magnitudes deemed useful and adjusting the current in the winding in order to regulate the slipping and provide the desired results with due consideration for the inherent characteristics of the generator.

This regulator may be of any known type, for example eiectromagnetic, electromechanical or electronic, as used in current practice for controlling the output of a variable-excitation generator, whether of the dynamo or alternator type.

In order to obtain a quicker response under transient conditions of operation, if the control member has a speed adapted to vary very rapidly, it may be advantageous to cause the regulation device to act at the same time upon the coupling winding and upon the generator winding. This result may be obtained by connecting in series or in parallel the winding 22 and the field winding of the generator between the source of current and the variable resistor of the regulator.

In FIGS. 6 and 7 of the drawings the winding 22 of of the coupling eleetromagnet is mounted in series with the field winding 1a of the generator. In FIG. 6 this mounting is utilized with a conventional voltage and current regulator 51 and 52 respectively, the voltage output of the generator being fed to the terminal 53.

In FIG. 7, the current for energizing the windings 1a and 22 is controlled by a transistor 54 having its base connected to the collector electrode of another transistor 55. The emitter of this other transistor 55 is connected to a Zener-rnounted semiconductor diode 56 or to any other suitable source of reference voltage. The voltage from the generator (either the output voltage of a dynamo or the rectified output voltage of an alternator) is fed to terminal 57.

In the examples set forth hereinabove, in case of rapid speed variation, for example toward the increase, the regulator simultaneously reduces the transmitted torque and the excitation while limiting more easily the generator output variations.

Similarly, a sudden increase in the generator output demand will simultaneously increase the transmitted torque and the generator excitation current, thus facilitating the application of load to the generator.

Permanent-magnet alternators, when intended for variable-speed operation, should be designed with a view to have a considerable field-Winding reaction and reactance in order to provide the necessary self-regulation as the speed increases, this solution being more economical than that consisting in controlling the output by means of an external variable reactance, for example in the form of a transducer, but permitting load variations only if the number of field windings operated in parallel is varied, this solution obviously involving complicated switching devices and being attended by many inconveniences.

The driving of the generator through an electromagnetic coupling, by permitting the operation of the former under constant or nearly constant speed conditions, facilitates the use and operation of machines of this character, as the output may be easily adjusted by combining the centrifugal regulation with the adjustment obtaining as a function of the output parameters.

In the example illustrated in FIG. 2, the driving pulley is secured on a shaft rigid with the bell-shaped element of the coupling by means of two ball-bearings having their outer races fitted in a bore of the front hearing.

In this case the ball-bearings revolve at the velocity of the driving pulley, thus limiting the maximum permissible speed of this pulley to the values consistent with the practical capacity of the bearings, that is, to a maximum speed of the order of 10,000 rpm. to guarantee a reasonable useful life thereof.

On the other hand in the arrangement proposed herein the field winding and the permanent magnet of the coupling are rigid with the generator armature and therefore some kind of rotary commutator device must be 7 provided for supplying energizing current to the coil, which requires an insulated commutator ring and a contact brush with the necessary guide members, springs, etc. Now these components are notoriously costly and subject to wear and tear.

In the form of embodiment shown in FIG. 8, a mechanical arrangement is proposed with a view to avoid the drawbacks pointed out hereinabove while reducing the weight of the coupling assembly.

In this figure the armature shaft '71 is journalled in a ball-bearing 73 fitted in a corresponding support 72 provided in the generator frame, the cylindrical extension 72a of this support carrying in addition the permanent magnet 74 and the field winding 75 connected on the one hand to the bearing ground and on the other hand, through an outlet conductor 76, to the field windings '77 of the generator. A pole piece 78 also secured on the support 72 constitutes the other pole of the electromagnet, the first pole consisting of the inner portion of the support 72 carrying the magnet 74 and the field winding 75.

The alternate-pole armature consists of two members 79 and 80. The member '79 is force-fitted and keyed on shaft '71 and supports the other member 80 through the intermediary of a piece 81 of non-magnetic material.

The pole-piece 79 receives its flux of one polarity through the gap formed between this piece '79 and the cylindrical extension 72a of support '72, and the other pole-piece 80 receives its fiux of opposite polarity through the gap formed between this piece 80 and the other pole piece 73, so that the usual pole shoes alternating at the periphery of pieces '79 and 80 will be successively NSNS etc.

The pulley consists of the outer side face of the bellshaped member 82 and of another pressed flange 83 secured on the hub 84 containing the ball-bearings 85 and $6 of which the inner races are force-fitted on the armature shaft 71. These bearings engage the pole piece '79 bearing in turn against the ball-bearing 73, washers 87 being provided to constitute distance-pieces maintaining the bearings at the proper relative spacing, the assembly being finally clamped by a nut 33.

The outermost ball-bearing 85 is disposed between two circlips 89 for properly positioning it in the axial direction, the outer race of the other bearing 86 being axially free so as to permit the expansion. The hub 84 is formed with two grooves containing rubber rings 90 having the dual purpose of holding the outer races against rotat on and taking up play so as to avoid any detrimental hammering effect between the outer races and the hub.

In order properly to lubricate the ball-bearings 85 and 86, these will preferably be of the type comprising a sealing ring on one side, such as the Model IRS manufactured by the SKF Company, the sealing rings being placed on the outer side of the bearings. Thus, a reserve of grease will be kept between the ball-bearings in the annular gap surrounding the distance-piece 87.

The generator and coupling are ventilated by means of axial-bladed fan 91 of annular configuration which is secured on the outer surface of the bell-shaped member 552 on the bearing side and constitutes a very efficient centrifugal fan drawing air through the generator in the usual manner through orifices 92 disposed at the periphcry of the bearing support 72 and between the poles of member 80 and the corresponding notches of distancepiece 81. This movable assembly 80, 81 constitutes on the other hand a primary centrifugal blade system so that the size of the external blade system 91 may be reduced accordingly.

In this modified embodiment given by way of example the field winding the permanent magnet of the electromagnetic coupling are secured on the bearing on the generator-driving side, and the pulley ball-bearings are mounted on the generator shaft instead of being supported by the bearing.

With this arrangement the rotary current feeding device may be dispensed with. On the other hand, the relative speed of the pulley ball-bearing races is only the difference between the pulley speed and the armature shaft speed.

By way of example, if we reconsider the case of a generator of which the speed ratio is 4:1 at its rated current output, and wherein the driving pulley rotates at a speed of from 2,500 to 10,000 rpm, with the novel a"- rangement proposed in this invention and in the case of a DC. generator a speed ratio of 2:1 may be adopted for the armature, which is practicable, the slipping ratio being 2:1 for the variable-speed coupling, so that the required total ratio of 4:1 is obtained without difiiculty.

Under these conditions it will be seen that with the combined regulation of the dynamo field winding and of the coupling coil as per above, the actual speed of the pulley bah-bearings will be only half the puiley speed. If we assume the case of the same dynamo delivering its rated output at 2,500 rpm, the armature will attain at the most a velocity of rotation of only 5,000 r.p.m., and at this time the pulley will revolve at 10,000 rpm. as well as the outer races of its ball-bearings, but as the inner races of these ball-bearings revolve at 5,000 r.p.1. i., the relative speed of the two races will only be l0,0005,000=5,000 rpm.

and therefore the useful life of these ball-bearings will be increased considerably.

Although this invention has been shown and described with specific reference to its application to generators of automotive electrical equipments, it will be readily understood by those conversant with the art that it is also applicable to other fields such as the equipments of variable-speed thermal engines, for example of the type utilized in the marine, in railway systems, public works, etc., and also to generator equipments utilized in railway cars wherein the generator is driven at a variable speed from the axle.

Besides, when carrying out the invention, modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

What I claim is:

l. A current generator equipment for automotive vehicles which comprises a driving shaft, a current generator comprising a stator and a rotor, a shaft of said rotor rotatably driven from said driving shaft, a variable-slip electromagnetic coupling disposed between said driving shaft and said rotor shaft of said current generator and comprising a driving rotor solid with said driving shaft, a driven rotor rigid with said rotor shaft of said current generator, an axial magnetic core, an annular permanent magnet carried by said hub and an annular winding carried by said hub and adjacent to said annular permanent magnet, means for varying the energizing current of said winding and therefore the magnetic flux produced by said winding as a function of the velocity of rotation of said generator rotor so that the degree of slip of said coupling varies as a function of the velocity of rotation of said driving shaft, said annular permanent magnet and said coupler winding being so arranged that their fluxes are additive and the magnetic flux produced by said permanent magnet be just sutlicient to rotatably drive said generator rotor when said winding is not energized.

2. Current generating equipment as set forth in claim 1, wherein said axial magnetic core carrying said permanent magnet and said annular Winding is rigid with said 9 MI? driven rotor of said coupling and said rotor shaft of said References Cited in the file of this patent gewator- UNITED STATES PATENTS 3. A generator equipment as set forth in claim 1, n

wherein said hub carrying said permanent magnet and said 7041573 Pfntsch y 15, 1902 Winding of said cougling is fixed, said driving shaft and 5 704,574 pllntsch y 15, 9 said armature shaft of the generator being disposed co 111881396 Binder June 1916 axially, the equipment further Comprising at least one 113381041 Rohman P 1920 rolling-contact bearing housed between said driving shaft 2,641,736 JaeSOhke lune 9, 1953 and said generator armature shaft and another bearing 2,732,921 Ra in W Jan. 31, 1956 housed between said hub and said generator armature 10 2,883,611 Fuge Apr. 21, 1959 shaft. 2,915,681 Troy Dec. 1, 1959 

1. A CURRENT GENERATOR EQUIPMENT FOR AUTOMOTIVE VEHICLES WHICH COMPRISES A DRIVING SHAFT, A CURRENT GENERATOR COMPRISING A STATOR AND A ROTOR, A SHAFT OF SAID ROTOR ROTATABLY DRIVEM FROM SAID DRIVING SHAFT, A VARIABLE-SLIP ELECTROMAGNETIC COUPLING DISPOSED BETWEEN SAID DRIVING SHAFT AND SAID ROTOR SHAFT OF SAID CURRENT GENERATOR AND COMPRISING A DRIVING ROTOR SOLID WITH SAID DRIVING SHAFT, A DRIVEN ROTOR RIGID WITH SAID ROTOR SHAFT OF SAID CURRENT GENERATOR, AN AXIAL MAGNETIC CORE, AN ANNULAR PERMANENT MAGNET CARRIED BY SAID HUB AND AN ANNULAR WINDING CARRIED BY SAID HUB AND ADJACENT TO SAID ANNULAR PERMANENT MAGNET, MEANS FOR VARYING THE ENERGIZATION CURRENT OF SAID WINDING AND THEREFORE THE MAGNETIC FLUX PRODUCED BY SAID WINDING AS A FUNCTION OF THE VELOCITY OF ROTATION OF SAID GENERATOR ROTOR SO THAT THE DEGREE OF SLIP OF SAID COUPLING VARIES AS A FUNCTION OF THE VELOCITY OF ROTATION OF SAID DRIVING SHAFT, SAID ANNULAR PERMANENT MAGNET AND SAID COUPLER WINDING BEING SO ARRANGED THAT THEIR FLUXES ARE ADDITIVE AND THE MAGNETIC FLUX PRODUCED BY SAID PERMANENT MAGNET BE JUST SUFFICIENT TO ROTATABLY DRIVE AND GENERTOR ROTOW WHEN SAID WINDING IS NOT ENERGIZED. 